Late Quaternary Slip Rates And Paleoearthquakes Along The Yabrai Range-front Fault In The Southern Gobi-Alashan Block

Study on the active faults of the southern Gobi-Alashan block is significant tounderstand the tectonic deformation processes associated with the Tibetan Plateau andOrdos block. With this knowledge in mind, the present thesis aims to answer what themajor process is governing the tectonic deformation and the structural relationshipsbetween Gobi-Alashan and adjacent regions. Since this area was featured by lateQuaternary active tectonics the activities and paleoearthquakes around the block needbe studied and reconciled. So, I will focus on one of those active structures, theranger-front fault along Yabrai Shan, to address some basic problems.Three aspects are analyzed, i.e., geomorphology, kinematic characteristics andpaleoearthquakes of the range-front fault along the Yabrai Shan. By combiningprevious studies and Differential GPS measurements, implications of the fault forthe regional tectonics has been discussed. The main conclusions are summarized asfollows:This fault consists of three segments. The most active segment is in thesouthwest. The southwest segment is about35km long. Its scarp, about1-2m high,might be the result of the most recent event (MRE) and stretches NE60°almost thefull segment. Existence of free surface indicates that the elapsed time of the last eventmust be not long.The middle segment is about31km in length. Just a single fault plane wasidentified along the main bounds of the Yabrai Shan, but the fault consists of severalsplays in the north. In contrast to the simple geometric structure of the middlesegment, the northeast segment comprises several faults. Scarps of the most recentearthquake event are clear but tend not to be continued. Field investigation reveals about0.5-1.5m up to2m height. Although the scarps along the southwest andnortheast segments along the fault are very similar, it is difficult to determine theorigin of the same earthquake without precise dating.This work determined slip rates by combining fault-scarp measurements andcosmogenic exposure age dating. Study on slip rates shows that the Holocene dip-sliprate of the Yabrai fault is0.11±0.03mm/a, and left-slip rates are between0.03±0.01mm/a-0.09±0.03mm/a. Scarp vertical heights along the fault show that the middleand north segments have obviously higher displacements than the southwesternsegment.Trenching and field survey along the surface ruptures indicate that there wereseven strong earthquakes along the range-front fault. The most recent event on thefault was about1.5ka ago, and the others were dated the late Pleistocene, i.e.,>120ka,>60ka,>50ka,~30ka,~3ka, respectively, with about10ka recurrence intervalof major earthquakes.A previous seismic reflection profile suggests that the Yabrai Shan range-frontfault was evolved as a normal fault since Cretaceous before the presence andformation of the Tibetan Plateau. Therefore, this work concluded that the range-frontfault of the Yabrai Shan is not resulted from the Indo-Asian collision. It should bestudied whether the outward growth of the Tibetan Plateau governs tectonicdeformation of the fault today, and which deformation model is suitable for theregions adjacent to Tibet, Ordos and Gobi-Alashan. This study would play a great rolefor constraining the tectonic evolution of the Tibetan Plateau.